1. Field of the Invention
The present invention relates to a wet-type image forming apparatus using electrophotographic technique and in particular to a squeezing technique for removing excess liquid developer from a photoconductor belt.
2. Description of the Related Art
In general, a wet-type image forming apparatus is composed of a squeezing roller arranged on the developed side of a photoconductor member and a backup roller arranged on the undeveloped side of the photoconductor member. The squeezing roller is used to remove excess liquid developer from the surface of the photoconductor member at the time of development of a latent image formed on the photoconductor member. The backup roller holds the photoconductor member with the squeezing roller under a predetermined pressure at the time of development so that the liquid developer remaining on the photoconductor member at the time of development is removed while at the same time reducing the thickness of toner on the photoconductor member.
After development, the force with which the photoconductor member is held by the squeezing roller and the backup roller is reduced to remove the liquid developer remaining between the squeezing roller and the photoconductor member (this operation is called "reverse squeezing"). In other words, upon complete development, the squeezing roller is driver in reverse rotational direction while kept in contact with the photoconductor belt 7 and a blade comes into contact with the squeezing roller to remove the residual ink.
An conventional apparatus for removing excess liquid developer on a photoconductor member has been disclosed in Japanese Unexamined Patent Publication No. 4-350878. This apparatus includes a means for detecting the rotational position of the photoconductor member and the squeezing roller, a means for calculating the gap under squeezing from the relation between the rotational position and the corresponding amount of eccentricity which are measure din advance, and a means for controlling the gap setting position of the squeezing roller to assure a constant gap with respect to the rotational position. The apparatus maintains a uniform gap between the photoconductor member and the squeezing roller and prevents the fluctuation of thickness of the toner layer on the surface of the photoconductor member.
A fixing unit including a heating roller and a pressure roller in contact with each other under a contact pressure appropriately controlled had been disclosed in Japanese Unexamined Patent Publication No. 8-160791. In the fixing unit, a transfer member such as a paper carrying an unfixed toner image thereon is passed through an area held between the heating roller and the pressure roller, so that the unfixed toner image is fixed by fusion on the transfer member with heat and pressure. The pressure between the rollers is controlled to change in accordance with the properly and thickness of the transfer member.
In the case where rollers are arranged parallel to the axes thereof, the shortage of the holding force develops in the central portion thereof, thereby leading to such inconveniences as an insufficient fixing and a conveyance failure at the central portion. A technique known to obviate the shortage of the holding force at the central portion consists in arranging the roller pair in a position with the axes thereof crossing each other. An excessively large crossing angle, however, undesirably causes the difference in the conveyance direction to develop a wrinkle of a thin transfer member. The cited patent publication discloses an apparatus for obviating this inconvenience. A cylindrical thin heating roller and a pressure roller are provided such that the heating roller is in contact with the pressure roller with crossing at a predetermined angle to a longitudinal direction. The crossing angle is adapted to be changed in accordance with the pressure of the pressure roller.
In the conventional wet-type image forming apparatus, as disclosed in Japanese Unexamined Patent Publication No. 4-350878, a squeezing roller and a backup roller are normally arranged with the axes thereof parallel to each other. In the case where a pressure is applied to the ends of the squeezing roller for exerting a large force (say, 6 kg to 12 kg) to hold the squeezing roller and the backup roller, the squeezing pressure exerted by the squeezing roller is larger at the axial ends thereof and smaller at the central portion along the axis thereof. This lack of uniformity of pressure leads to a problem of variations of the image drying rate, thereby causing a nonuniform transfer efficiency to the paper.
In the case where the pressure on the backup roller is increased for correcting the imbalance of the squeezing pressure, the squeezing roller is prevented from being smoothly driven so that the image is liable to be scraped off.
A method for securing a uniform squeezing pressure at the time of development by crowing or tapering the backup roller, on the other hand, has a disadvantage that the squeeze pressure is liable to develop an imbalance at the time of reverse squeeze.